The present invention relates generally to sensors for electronically sensing the presence of an object and generating a switching signal in response to the object, and in particular to such a sensor having improved noise immunity.
Many switching circuits and devices have been proposed and are currently in use. In certain switches, the presence of an object, such as the hand of an operator, a workpiece, and so forth, is sensed and serves as a basis for changing the conductive state of a switch. The presence or absence of an object may be detected by analyzing the interaction between the object and an electronic sensor. For example, the sensor may detect changes at a sensing surface, caused by an object touching the sensing surface. Sensing in such cases may be based upon changes in temperature, electrical resistance, radio-wave reception, electrical capacitance or inductance, and so forth. In electronic sensors, sampling is commonly used to detect signals that can be processed to determine whether the monitored parameter has changed, and that an output signal is warranted. Electrical noise from the environment, however, may interfere with the sensor's sampling of data and may result in faulty operation of the sensor.
In certain sensor designs, the presence or absence of an object may be detected by measuring the interaction of the object with an electromagnetic field generated near the sensing surface. The object, when near or touching the sensing surface, introduces a new or changed impedance into the circuit generating the electromagnetic field through capacitive or inductive coupling. In a capacitive presence sensor, for example, an object may increase a capacitive coupling between an electrode of the generating circuit and environmental ground return paths. In an inductive presence sensor, the object may inductively couple to an antenna of the generating circuit to change the effective inductance of the antenna.
This change in impedance, caused by the introduction of an object near or touching the sensing surface, is manifested as an energy transfer from the generating circuit to the object, such energy transfer being detected by a sensing circuit, for example, as increased current flow. The amount of energy transfer may be compared against a threshold to produce a binary, switched output indicating the presence or absence of an object. In certain applications, it may be desirable for the sensor to be a proximity device, or in other words, to only require the sensed object enter a sensing volume. In other applications, it may be desirable for the sensor to be touch-sensitive, or in other words, to require the sensed object touch the sensing surface encompassing the electrode of the generating circuit.
Electromagnetic field presence sensors may be used in a wide variety of consumer and commercial applications. For example, touch-sensitive lamps found in homes employ a capacitive presence sensor. Electromagnetic field presence sensors are particularly useful in hostile industrial environments because the sensors do not require physical or electrical (ohmic) contact with the object, and can be easily sealed against water and dirt. Moreover, these sensors are particularly useful in industrial automation and control systems. For example, in an industrial context a human hand placed in the sensing volume or on the sensing surface can actuate functions of machinery. The use of inductive and capacitive presence sensors to control machinery reduces human operator fatigue associated with repetitive control switch actuation (i.e., as with traditional electromechanical push buttons). Over the past several years it has become increasingly evident that repeated actuation of electromechanical switching devices, such as in assembly lines, production machinery, and the like, can lead to operator fatigue.
While the devices greatly reduce fatigue by relieving the operator of the need to depress or move a mechanical actuator, presence sensing devices have been prone to false actuation by sources other than the object of interest (e.g. an operator's hand). Particularly problematical sources of actuation include moisture, machinery fluids, various articles, tools, and so forth. Additionally, electrical noise from the environment or conducted through power or even data lines may cause false triggering of the sensor, particularly when sampling techniques are used as a basis for determining the presence of an actuating object.
A particular difficulty in sampled data sensing systems is the presence of periodic or cyclic noise. The detection circuitry typically determines whether a detected signal has persisted for a preset duration or number of sample periods, and such periodic noise, if corresponding to the period of sampling, can produce data appearing to indicate the presence of an actuating object in error. Averaging circuitry may be added to the sensing circuitry so as to diminish the effect of noise in such cases. Such averaging circuitry, however, has drawbacks, such as slowing the response of the presence sensor to changes in the presence or absence of an object it is detecting, thus limiting the application of such switches in cases where fast response is required.
There is a need, therefore, to improve the reaction time of electromagnetic field presence sensors, to reduce the potential for false actuation of these sensors, and more particularly, for these sensors to reliably differentiate between bare or gloved hands and foreign objects. A particular need exists for these sensors and switches that exhibit reduced noise sensitivity, particularly to periodic or cyclic noise. Furthermore, there is a need for such sensors and switches capable of self-calibrating and detecting when conditions are outside normal operating ranges.